The incorporated parent disclosure (Part I) is generally related to cyclic monomers bearing pendant pentafluorophenyl ester groups for ring-opening polymerizations, and methods of preparation thereof. The continuation-in-part disclosure (Part II) is generally related to polymers bearing pendant pentafluorophenyl ester groups, methods of their synthesis, and methods of their functionalization; and more specifically to polycarbonates bearing pendant pentafluorophenyl ester groups and their functionalization.
Part I. Background
In general, the structural variety of monomers for ring opening polymerization (ROP) is significantly less than the number of monomers available for controlled radical polymerization (CRP). However, as the effectiveness and operational simplicity of organocatalysts improves, a wider variety of ROP monomers is sought to generate polymer microstructures unique to ROP methods.
Initial efforts to employ substituted lactones as ROP monomers were hampered by the sensitivity of the organocatalysts to steric bulk of the monomer, particularly at the alpha-position. The alpha-position of cyclic esters is the only site capable of a general substitution reaction. Consequently, this approach provided limited numbers of monomers. More encouraging was the finding that trimethylene carbonate (TMC) was efficiently polymerized by organocatalysts such as thiourea/1,8-diazabicyclo[5.4.0]undec-7-ene (TU-DBU) or 1,5,7-triaza-bicyclo[4.4.0]dec-5-ene (TBD), for two reasons: first, TMC-like monomers can be derived from readily available 1,3-diols, and second, those 1,3-diols can be chosen so as to only bear substituents at the 2-position (the 5-position in the cyclic carbonate), where the substituent does not interfere sterically with the ring opening polymerization.
A number of cyclic carbonate monomers have been generated and polymerized in the past by more conventional anionic or organometallic ROP methods. Excessively bulky substituents (e.g., 2,2-diphenyl) in the 1,3-diol can make ring-opening of the corresponding cyclic carbonate thermodynamically unfavorable. Thus, efforts were focused on monomers derived from 2,2-bis(methylol)propionic acid (bis-MPA), a common building block for biocompatible dendrimers. Cyclic carbonate monomers have been generated from bis-MPA with a number of different functional groups attached to the carboxylate, usually involving methods similar to those used in dendrimer synthesis (i.e., acetonide protection and deprotection followed by carbonate formation). Scheme 1 illustrates known synthetic routes to functionalized cyclic carbonyl compounds from bis-MPA (Pratt et al. Chem. Comm. 2008, 114-116).
The following conditions apply to the reactions in Scheme 1: (i) benzyl bromide (BnBr), KOH, DMF, 100° C., 15 hours, 62% yield. (ii) triphosgene, pyridine, CH2Cl2, −78° C. to 0° C., 95% yield. (iii) Pd/C (10%), H2 (3 atm), EtOAc, RT, 24 hours, 99% yield. (iv) ROH, dicyclohexylcarbodiimide (DCC), THF, room temperature (RT), 1-24 hours. (v) (COCl)2, THF, RT, 1 hour, 99% yield. (vi) ROH, NEt3, RT, 3 hours.
The cyclic carbonate acid monomer, MTC-OH, (1),
provides great versatility in preparing functionalized carbonate monomers for ROP, similar to meth(acrylic) acid for CRP (see Pratt et al.). For example, the reaction of an alcohol or amine with an active ester of a (meth)acrylic acid provides a (meth)acrylate or (meth)acrylamide monomer for CRP. Likewise, the reaction of an arbitrary alcohol or amine with an active ester of MTC-OH can generate a cyclic carbonate ester or cyclic carbonate amide monomer for ROP.
Two procedures for esterifying MTC-OH are typically employed: a) direct coupling of MTC-OH with an alcohol using dicyclohexylcarbodiimide (DCC); or b) conversion of MTC-OH to the acyl chloride with oxalyl chloride followed by reaction with an alcohol (ROH, wherein R is a substituent comprising 1 to 20 carbons) or amine in the presence of base, as shown in Scheme 1. The latter method has the advantage that the salt byproducts are easily removed; however, the acyl chloride intermediate is extremely water sensitive which presents storage and handling concerns. In addition, both procedures are labor and resource intensive, use significant amounts of solvent and reagents, and are not environmentally “green.”
“Green” chemistry is a concept that is being embraced around the world to ensure continued economic and environmental prosperity. Modern synthetic methodologies are encouraged to preserve performance while minimizing toxicity, use renewable feedstocks, and use catalytic and/or recyclable reagents to minimize waste. Green chemistry is the design and development of chemical products and processes that reduce or eliminate the use of substances harmful to health or environment.
Part II. Background
Biodegradable polymers are of intense for use in a variety of nanomedicine applications including drug delivery/target therapeutics, imaging agents, and tissue engineering. The two most common approaches to the synthesis of biodegradable polymers are the ring-opening polymerization (ROP) of cyclic esters (e.g., lactones) and cyclic carbonates to produce polyesters and polycarbonates, respectively, illustrated in Scheme A.
wherein R1 and R2 generally represent hydrogen or a short chain monovalent hydrocarbon substituent, and n is 1 to 5. As a class of biodegradable polymers, polycarbonates have generally been found to exhibit significantly increased rates of biodegradation in the human body relative to polyesters.
Cyclic carbonate monomers based on MTC-OH require that the carboxylic acid group be protected (most commonly as a benzyl ester) or functionalized prior to ring opening polymerization, as shown in Scheme B.
The protected/functionalized monomer is then homopolymerized or copolymerized with other cyclic carbonate or cyclic ester monomers (e.g., lactide and/or epsilon-caprolactone). After polymerization the benzyl protecting groups can be removed by hydrogenolysis, to form side chain carboxylic acid groups. The carboxylic acid groups can then be converted to esters or amides with a suitable nucleophile, R—XH, using known coupling chemistry (e.g., see Jing et al., J. Appl. Polym. Sci. 2008, 110, 2961-2970), wherein R—XH represents an alcohol, an amine, or a thiol. Alternatively, molecules can be coupled to the polycarbonate using various other coupling reactions such as Diels-Alder reactions, 1,3-dipolar cycloadditions, and thiol-ene reactions. In these cases, a reactive functional group (e.g., a group containing a diene, azide, alkyne, or alkene functionality) is attached to the polymer backbone through a pendant ester/amide linkage. The reactive functional group can be attached to the monomer prior to polymerization (i.e., during monomer synthesis) or to the polymer after polymerization. An appropriately functionalized cargo molecule (e.g., comprising a dienophile, an alkyne, an azide, or a thiol functionality) can then be coupled to the polymer by reaction with the pendant reactive functional group. Examples using propargyl or allyl functionalized cyclic carbonate monomers are described by Jing et al., in Biomaterials 2008, 29, pgs. 1118-26; Macromol. Biosci. 2008, 8, pgs. 638-644; J. Poly. Sci. A: Polym. Chem. 2007, 45, pgs. 3204-3217; and J. Poly. Sci. A: Polym. Chem. 2008, 46, pgs. 1852-1861. Examples in which furan- or azide-functionalized groups are attached to the pendant carboxylic acid groups of the polycarbonate are described by Shoichet et al., in Bioconj. Chem. 2009, 20, pgs. 87-94; J. Biomat. Sci. Polym. Ed. 2008, 19, pgs. 1143-57; Angewandte Chem. Int. Ed. 2007, 46, pgs. 6126-6131; and WO 2007/003054A1. These approaches require significant numbers of synthetic steps to incorporate the required reactive functional groups into the cyclic carbonate monomer/polycarbonate, as well as the cargo molecule.
Alternatively, Zhuo et al., Macromol. Rapid Commun. 2005, 26, pgs. 1309, demonstrated the copolymerization of, and subsequent functionalization of, a cyclic carbonate monomer bearing a reactive pendant succinimidyl ester. The synthesis of this monomer required 4 steps and afforded only a low yield (˜20%). In addition, copolymers made with the succinimidyl ester-functionalized cyclic carbonate had broad polydispersities (PDI 1.8-2.9) indicating that this chemistry may be unsuitable for the synthesis of materials having tailored molecular architectures. Most problematically, attempts to polymerize this monomer using organic catalysts were unsuccessful. One reason for this is the insolubility of the monomer at room temperature in solvents commonly used for ring opening polymerizations (e.g., toluene), which required the use of DMSO.
As a result of the aforementioned limitations of the known art, a more versatile and straightforward approach to the preparation of ROP polymers bearing reactive side chain groups is needed, in particular polycarbonates bearing reactive side chain groups. The reactive side chain groups should enable direct functionalization of the ROP polymer.